Bisphenol-A (4,4′-dihydroxy-2,2-diphenylpropane or BPA) is produced by condensation of acetone with an excess of phenol in the presence of an acidic catalyst or a cation-exchange resin. The crude product, in addition to the desired bisphenol-A and unreacted phenol, contains unwanted by-products, such as bisphenol-A isomers, trisphenols and other higher molecular weight materials. The bisphenol-A is normally separated from the crude product by a single or a series of crystallization steps, leaving a mother liquor stream enriched in unwanted by-products, a portion of which stream is removed to purge unwanted by-products from the process. Alternately, the bisphenol-A may be separated from the crude product by a single or series of distillation steps, which also creates a stream enriched in unwanted by-products, a portion of which is removed. The removed stream may contain unreacted phenol and bisphenol-A as well as the unwanted by-products. Phenol is typically recovered from the removed stream by distillation, normally vacuum distillation.
In order to increase recovery of phenol for recycle into the BPA process, it is known to catalytically crack the unwanted by-products, many of which contain phenol-moieties within their structures. Typically, the products of the cracking step are recycled into a distillation step, where the heat from the cracking process helps to provide the heat input required by the distillation step and the phenol liberated in the cracking step is recovered. The products of the cracking step are typically phenol or a combination of phenol and isopropenyl phenol (IPP). In this process, the system uses the cracking heat source to supply both the cracking heat duty and the distillation column reboiler duty required for phenol recovery. U.S. Pat. No. 6,459,004 to Ono et al. discloses such a system (col. 2, lines 31-36).
However, the distillation step does not require as high a temperature as the cracking reaction, but the reboil heat duty required to recover the products of the cracking step is substantially higher than the heat duty required for cracking. The result is that a higher quality (higher temperature) and therefore higher cost heat source is used to supply a much larger heat duty than is required, which is not economically efficient.